In looking through the MIB, there seems to be no similar Net-SNMP data to indicate if a device is a real, physical media device. The closest I could find was the physical device path, which starts with /dev/ for real devices. I added that as suggested change below:

<threshold
description="Trigger an alert when the percentage of inodes used on any disk with device name starting with '/dev/' (that is, an actual physical device) reaches or passes 90% for two measurement intervals"
type="high" ds-type="dskIndex" value="90.0" rearm="75.0"
trigger="2" ds-label="ns-dskPath"
filterOperator="or" ds-name="ns-dskPercentNode">
<resource-filter field="ns-dskDevice">^/dev/.+$</resource-filter>
</threshold>

The "relativeChange" threshold on the same value should have the same filter applied.